I am planning to use ISP programmer to program my new AVR board. I use an 8 MHz crystal on my PCB to clock the micro (atmega328p-au). Now, as I understand the ISP programmer will provide its own clock to my board when programming. It occurs to me that this might pose an issue. I've read that using an SPI clock much slower than the system clock is the way to go, but haven't found any specific documentation. Any suggestions? Thanks!
ISP clock can't "interfere" with on-board clock. ISP clock is the clock for the ISP data. The MCU still needs a clock to run itself, and this clock needs to be high enough for the given ISP clock. Slower MCU clock needs slower ISP clock for sure, exact parameters are given in AVR datasheet.